Integrated circuits generally contain millions of individual electronic devices, each typically the size of a few micrometers, and interconnections thereamong. As a result, no "physical" tool is adequate for fabricating the complex patterns representing the devices and interconnections on a semiconductor substrate. Instead, microelectronic patterning is conventionally performed by radiation, e.g., light, x-rays or electron beams. The process of using an optical image and a photosensitive film to produce a pattern on a semiconductor substrate is known as "photolithography."
In photolithography, a film of a photoresist (photosensitive film) is first applied to the substrate. Radiation is then projected through a transparent plate, or "mask," on which has been created a desired pattern in an opaque material. The resulting image is focused on the photoresist-coated substrate, yielding areas of light and shadow on the substrate that correspond to the image on the mask plate. Following development and etching processes, where portions of the resist and substrate are removed, a pattern corresponding to that on the mask is etched in the substrate film.
During the infancy of microelectronics, the master image (pattern) was photographically reduced from a macroscopic original. The desired pattern was cut by hand into a colored plastic sheet and the image reduced to the desired size using room-sized reduction cameras. This method has been replaced by a pattern generator, an apparatus that accepts a computer-generated description of the device and analyzes it into individual picture frames. The pattern generator then scans a mask plate, "writing" the pattern on the mask plate using, for example, a high intensity electron beam, to expose each device or interconnect in the pattern.
Semiconductor devices may typically be made up of as many as fifty individual layers of silicon, polysilicon, silicon dioxide, metal and silicides. The pattern for each layer is contained on a mask called a reticle. Reticles are generally between one and ten times the actual size of the pattern they produce. The group of reticles that corresponds to all the layers of an integrated circuit (IC) is called a reticle set.
The pattern generation process, requiring point-to-point creation of a complex pattern on a semiconductor wafer, is generally a slow process. Usually only one of the multiple device images to be printed on a semiconductor substrate is pattern-generated. The single device pattern, or reticle, is then replicated repeatedly using a step-and-repeat camera (commonly known as a stepper) to form the array of devices and interconnects that cover the semiconductor substrate. The reticle may also consist of an array of several patterns and is distinguished from a mask that contains patterns that can be transferred to an entire semiconductor wafer (or to another mask) in one exposure.
Not only is the production of a reticle a slow process, it is also an expensive process. For example, it may take up to several hours for a conventional electron-beam exposure apparatus to produce a reticle with a circuit pattern of a degree of integration equivalent to that of a 256 MDRAM integrated circuit. As mentioned-above a reticle set corresponding to a semiconductor device that has up to fifty reticles is not uncommon. Therefore, with the current electron-beam exposure apparatus production rate limited to only be a few reticles a day, the production of the semiconductor device may be significantly delayed.
Accordingly, what is needed in the art is an improved method to manufacture a reticle that overcomes the above-described limitations.